The present invention relates generally to a method of joining ceramic to metal and specifically to a method of creating a ceramic-metal interface that allows torsion loads to be transmitted between a ceramic member and a metal member. The invention also relates to turbomachinery including metal and ceramic rotating components that are joined together.
One of the more difficult problems of joining ceramic to metal is overcoming the large difference in thermal expansion between the metal and the ceramic. In particular, a ceramic such as silicon nitride has a low thermal expansion as compared to a steel or nickel alloy.
Interlayer materials including tungsten alloys have been used to overcome the difference in thermal expansion. The interlayer materials reduce residual stresses produced by the large, differences in thermal shrinkage upon cooling after brazing.
In an automotive turbocharger including a ceramic turbine wheel that is brazed to a metal shaft, a metal sleeve is used to cover the braze joint. The metal sleeve is typically made of a special low thermal expansion metal alloy that avoids the introduction of unwanted residual stresses in the ceramic. The metal sleeve protects the braze joint from high temperatures, and it protects the braze joint from cracking under high bending and twisting loads. The metal sleeve also provides a sealing surface. (A ceramic shaft of the turbine wheel extends from a hot side of the turbocharger, through a seal, to a cooled side. However, it is undesirable to form a seal on a brittle ceramic shaft.) Additionally, the metal sleeve itself can provide additional bonding, as is the case where the ceramic shaft is press-fitted into the metal sleeve.
However, special metal alloys used for the metal sleeve are expensive and they are not readily available. Furthermore, precision machining of the ceramic and metal mating surfaces is performed to avoid the introduction of unwanted stresses and, in the case of press-fitting, to ensure that the mating surfaces remain in contact throughout the operating temperature range of the machine. The sleeve also makes a post-brazing inspection of the braze joint difficult to perform. These problems are especially important to mass production items such as automotive turbochargers.
Furthermore, brazing is often performed in two steps when a metal sleeve is involved. The addition of a second step adds to the cost and complexity of joining the metal to the ceramic.
Even when a metal sleeve is not involved, some brazing operations are performed in two steps. The metal is brazed separately from the ceramic to prevent a migration of braze material between layers that might otherwise occur during brazing of multiple materials. The migration could cause a change in composition and properties of the braze and thereby have a deleterious affect on strength of the joint.
There is a need to join metal to ceramic in a single brazing step. In applications involving the transmission of large torsion loads, there is also a needed to create a high strength, sleeveless braze joint.